Abstract: The proposed study will elucidate regulatory controls in the replication of mitochondrial DNA. Replicative forms will be isolated from Thymidine kinase minus lines of mouse cells. Control of replication at the level of transcription will be examined through the use of Dark-field Electron Microscopy of purified transcription complexes and by density gradient centrifugation of pulse-labeled components. Mitochondrial DNA replication will be stimulated by release of the tissue culture cells from stationery growth or infection with oncogenic viruses. Transcriptional control through RNA priming of initiation will also be examined. Regulation mediated through mitochondrial specific DNA nucleases in displacement replication will be detailed and the responsible enzymes will then be purified. Parallel studies will be conducted with circular dimer mitochondrial forms purified from human leukemic white cells before and after patients have been treated with antileukemic drugs. Control at initiation of displacement replication and the general validity of the displacement loop model for mitochondrial DNA replication will be tested through labeling studies on Thymidine kinase minus cells. The sites on mitochondrial DNA where regulatory elements function will be determined relative to known-mitochondrial genes by the techniques of electron microscopy.